Liquid chromatography (LC) is a well known technique for separating the constituent elements in a given sample. In a conventional LC system, a liquid solvent (often called the "mobile phase") is introduced from a solvent reservoir and delivered via tubing to a pump. Many different types of conventional LC systems and components for LC systems are commercially available from a number of vendors. For example, Millipore Corporation of Milford, Mass., Beckman Instruments of Fullerton, Calif., and Hewlett-Packard Company of Palo Alto, Calif., all sell LC systems, including pumps, sample injection valves, columns, and detectors, among other things.
In operation, a pump in the conventional LC system creates a vacuum which sucks the solvent through the tubing (suction tubing) and into the pump. An inlet solvent filter assembly is connected to the suction tubing leading from the solvent reservoir to the pump. This filter assembly is placed directly in the solvent reservoir. The purpose of the inlet filter is to remove any particles from the solvent and prevent the particles from reaching valves in the pump. A secondary function of the inlet filter is to act as a "sinker" to hold the suction tube inlet at the bottom of the solvent reservoir, thereby avoiding the introduction of air into the suction tube and/or the pump.
In a conventional LC system, the solvent exits the pump under a higher pressure and then passes to the sample injection valve. As the name suggests, the sample injection valve allows an operator to inject a sample into the LC system, where the sample will be carried along with the mobile phase. After the sample injection valve, most conventional LC systems include a column. A typical column usually consists of a piece of steel tubing which has been packed with a "packing" material. The "packing" consists of the particulate material inside the column. Conventional packing materials usually consist of silica- or polymer-based particles, which are often chemically bonded with a chemical function. When the sample is carried through the column (along with the mobile phase), the various components (solutes) in the sample migrate through the packing within the column at different rates (i.e., there is differential migration of the solutes). Because of the different rates of movement, the components gradually separate as they move through the column. A more detailed description of the separation process can be found, among other places, in Chapters 2 and 5 of Introduction to Modern Liquid Chromatography (2d ed. 1979) by L. R. Snyder and J. J. Kirkland, which chapters are incorporated by reference herein.
Once the sample (with its components now separated) leaves the column, it flows with the mobile phase past a detector. The detector detects the presence of specific molecules or compounds. As discussed in Chapter 4 of Introduction to Modern Liquid Chromatography, which chapter is incorporated by reference herein, two general types of detectors are used in conventional LC applications. One type measures a change in some overall physical property of the mobile phase and the sample (such as their refractive index). The other type measures only some property of the sample (such as the absorption of ultraviolet radiation). In essence, a conventional detector in an LC system can measure and provide an output in terms of mass per unit of volume (such as grams per milliliter) or mass per unit of time (such as grams per second) of the sample's components. From such an output signal, a "chromatogram" can be provided; the chromatogram can then be used by an operator to determine the chemical components present in the sample.
In addition to the above components, a conventional LC system will often include filters, check valves, and the like in order to prevent contamination of the sample or damage to the LC system. It will be understood to those skilled in the art that, as used herein, the term "LC system" is intended in its broad sense to include all apparatus used in connection with liquid chromatography, whether made of only a few simple components or made of numerous, sophisticated components which are computer controlled or the like.
In the past, filter assemblies often included filtering elements which required specially designed fittings for the suction tube connection. These prior art filters either had a stem for the tubing connection or a flangeless fitting which received a threaded flangeless nut on the end of the tube. Because inlet filters eventually become clogged with particle build up, they need to be changed from time to time. Hence, the special design of such prior art filters make them expensive to replace when they need changing. In addition, these prior art filters require a great deal of time to change since the suction tube must be disconnected from the filter cup assembly and then reconnected to a new filter cup. Such wasted time is often at a premium because of the expensive nature of many LC systems; changing such filters often renders the entire LC system inoperative until the change was completed.
Another problem with many conventional inlet filters is that they do not draw solvent from the bottom of the solvent reservoir. The use of such filters often results in the waste of expensive solvents. One conventional filter which is designed to draw from the bottom of the reservoir is shown in Bulletin #197 of Altech Associates, Inc., which is incorporated by reference herein. The suction tubing connected to this particular filter assembly draws fluid through a flat, disk shaped filter element (i.e., a "frit"). This flat filter element has a limited filter surface area, and therefore tends to clog relatively quickly. Moreover, the frit cannot be easily replaced when it becomes clogged. Hence, such conventional filters lead to shorter time periods between filter changes and require the replacement of the entire filter assembly. Also, such filters do not have any mechanism to allow an operator to quickly disconnect the filter assembly in order to change the filter element. Because the filtering elements themselves cannot be easily replaced by an operator, the entire filter assembly must be discarded. Hence, such filters clog relatively quickly and, when they do, they must be replaced by an entirely new filter assembly, a costly and inefficient approach to providing a solvent filter.
Accordingly, it is an object of the invention to provide an improved inlet filter which is more efficient and uses replaceable filtering elements.
Another object of the invention is to provide an improved inlet filter wherein the filtering element can be more easily replaced by an operator.
Another object of the invention is to provide an improved inlet filter which uses a replaceable filtering element.
Still another object of the present invention is to provide an improved inlet filter in which the filtering element can be replaced without disconnecting the rest of the filter assembly from the suction tube.
Still another object of the present invention is to provide a cheaper inlet filter in which the filtering element can be easily replaced without discarding the entire filter assembly.
A further object of the invention is to provide an improved inlet filter which positions the inlet of the tube through which the filtered liquid is drawn near the bottom of the liquid reservoir.
Still another object of the invention is to provide an improved inlet filter which will assist an operator in positioning the suction tube inlet at the bottom of a liquid reservoir.